Superheat - 2011-05

Measuring Superheat

When a refrigerant is heated above its boiling point it is said to be superheated. If the refrigerant expansion valve is closed too far, there will be too much superheat and the compressor will run hot which is inefficient. If the expansion valve is opened too far, there is too little superheat and the compressor will run too cold and possibly flood.

The superheat adjustment process should be done when the freeze dryer is at a no-load condition. That is to say, the system is running as cold as you would ever normally operate the system.

1. Ensure that the sensing bulb for the thermo expansion valve (TXV) is firmly clamped to the suction line on a horizontal pipe at about the 10:00 or 2:00 o'clock position.

2. Remove the protective cap covering the superheat adjustment screw.

3. Securely attach your thermometer probe to the suction line as close as you can to the location of the bulb of the TXV.

4. Read the suction pressure at the compressor with the compressor running and the system temperature stabilized.

5. Determine the evaporator temperature. Do this by reading your suction pressure displayed on the gauge on the freeze dryer. Use your refrigerant temperature/pressure chart to determine the actual evaporator temperature.

6. Measure evaporator exit temperature. Use your surface reading thermometer to read the temperature of the suction line at the exit point of the heat exchanger.

7. Calculate superheat. (see instructions below)

8. The correct superheat settings on a freeze dryer are usually between 5-deg and 7-deg F in a no-load condition.


Calculating Superheat 

1. Measure the temperature of the suction line at the point where the expansion valve bulb is attached to the suction line.

2. Determine the suction pressure in the suction line by using the refrigeration gauge.

3. Convert the suction pressure, in step 2, to the saturated evaporator temperature. Using the pressure/temperature chart for the refrigerant you are using convert the suction pressure to temperature.

4. Subtract the temperature obtained in step 3 from the temperature in step 1. The difference will be the superheat temperature.

5. This can be done at any point in the system.


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Tech Tips Home

These technical tips are for use by qualified maintenance personnel who are familiar with their specific freeze drying equipment. These tips are intended only as general guidelines. Lyophilization equipment is frequently custom configured and some tips may not be appropriate for all freeze dryers. Always read and follow the directions of your equipment's maintenance manual. If you would like to discuss one of our tech tips, please contact Dave Clayton at 215-672-7800 ext-1376.


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